The present invention is concerned with improving braking performance in vehicle braking systems, in particular within the context of vehicle braking systems having electro-hydraulic (EHB) braking.
A typical EHB system for a vehicle comprises a brake pedal, respective braking devices which are connected to the vehicle wheels and which are capable of being brought into communication with electronically controlled proportional control valves in order to apply hydraulic fluid under pressure to the braking devices, a hydraulic pump driven by an electric motor, and a high pressure hydraulic pre accumulator fed by said pump for the provision of hydraulic fluid under pressure which can be passed to the bra device via the proportional control valve in order to apply hydraulic fluid under pressure to the braking devices in so called xe2x80x9cbrake-by-wirexe2x80x9d mode in proportion to the driver""s demand as sensed at the brake pedal. The EHB system is controlled by an electronic controller (ECU).
In order to enable the vehicle to be braked in conditions where for some reason, the ERB system has become inoperative, for example because of a major component failure, it is usual in vehicles fitted with EHB to include a mechanical back-up system comprising a master cylinder which is linked to the brake pedal and which can be arranged to be coupled hydraulically to respective brake actuators at the front wheels to provide at least some braking in the event of total EHB failure. This is known as the xe2x80x9cpush-toughxe2x80x9d mode of braking. In order to make the PM system xe2x80x9cfeelxe2x80x9d like a conventional braking system in the xe2x80x9cpush-throughxe2x80x9d mode of braking, a travel simulator is also usually provided which is connected hydraulically to the master cylinder coupled to the brake pedal and which allows, by increasing the volume under pressure, the brake pedal to be depressed to an extent comparable with that of conventional systems.
The performance of an electro-hydraulic braking system (EHB) in its back-up (push-through) operating mode is, however, not as good as when the EHB is working normally in that there is no braking at all at the rear axle and no booster function for the action of the hydraulic cylinder.
Some prior art systems provide 4-wheel push-through, but do not isolate the power-circuit fluid from that in the hydrostatic push-through circuit. This makes the push-through function vulnerable to fluid aeration.
One could in principle provide 4-wheel push-through braking, but only by using two more isolation pistons, two more isolation solenoid valves and a larger master cylinder. However, this would be unacceptable due to the cost, packaging and consideration of additional heat dissipation and electrical energy consumption.
Some vehicles are now fitted with electrically operated parking brake systems (EPB) wherein for paring purposes, the normal braking devices, in addition to being actuable hydraulically, can be brought into a braking condition electrically. For example, the brake actuator can include a piston which is drivable by a reversible electric motor to apply and retract the brake shoes from a brake disc for respectively applying and releasing parking braking.
In accordance with the present invention, there is provided a vehicle braking system comprising an electro-hydraulic braking means of the type which operates normally in a brake-by-wire mode wherein hydraulic pressure is applied to braking devices at the vehicle wheels in proportion to the driver""s braking demand as sensed electronically at a brake pedal, and which, if the brake-by-wire mode should fail, operates in a push-through mode wherein hydraulic pressure is applied to the braking devices at the vehicle wheels by way of a master cylinder coupled mechanically to the brake pedal, and an electric parking braking means for enabling the braking devices to be actuated for parking braking purposes, wherein for supplementing the push-through braking provided by the electro-hydraulic braking means in the event that the brake-by-wire mode had failed, it is arranged that the operation of the foot pedal by the driver also causes operation of the electric parking braking.
To achieve this, an ECU within the EPB system should preferably receive a signal indicating the operational status of the EBB system. With such an arrangement, whenever the EHB is working correctly, then the EPB would receive the EHB status signal and respond only to the normal paring-brake control. On the other hand, if the EBB status signal is not received by the ECU of the EPB, indicating that the EBB is not operational then the parking braking will be allowed to respond, not only to the parking brake control but also to the operation of the brake pedal. Braking will then be obtained from both the push-through operation and EPB.
In some embodiments of such a system, there could be the problem that failure of the status signal link between the ECUs in the EHB and the EPB, for example by way of an open-circuit connector fault, could allow the EPB to operate in response to brake pedal signals at times when the EHB was actually still operational. The additional brake torque resulting from both systems operating together could cause severe overbraking at the rear axle, with a consequential risk of vehicle instability.
To overcome this problem, it is preferred that electro-hydraulic braking at the rear axle of the vehicle is allowed only when a control unit of the electro-hydraulic braking means has confirmation that the electric parking means is in a satisfactory operational state.
The system can include electronic control units for controlling electro-hydraulic braking and electric parking braking, respectively, which are interconnected such that electro-hydraulic braking at the rear axle of the vehicle is allowed only when the control unit for electro-hydraulic braking has confirmation from the control unit for electric parking braking that the electric parking means is in said satisfactory operational state.
Advantageously, the control unit for electro-hydraulic braking is arranged to provide a fit status signal to the control unit for electric parking braking whereby whenever a status signal indicative of the electro-hydraulic braking means being in a satisfactory operational state is received by the control unit for electric parking braking, the electric braking means responds only to parking braking control.
Also advantageously, the control unit for electric parking braking is arranged to direct a second status signal to the control unit for electro-hydraulic braking for providing said confirmation that the electric parking braking means is in said satisfactory operational state.
Preferably, said first and second status signals are transferred between the control units via a common link whereby if the link itself fails, neither status signal is transferred between the two control units.